1. Field of the Invention
The present invention relates to a method and apparatus for treating wastes by gasification, and more particularly to a method and apparatus for treating wastes by gasification at a relatively low temperature and then at a relatively high temperature to recover useful resources including energy, valuables such as metals, and gases for use as materials for chemical industries or fuel.
2. Description of the Prior Art
It has heretofore been customary to treat a considerable amount of wastes such as municipal wastes, waste tires, sewage sludges, and industrial sludges with dedicated incinerators. Night soil and highly concentrated wastes have also been treated with dedicated wastewater treatment facilities. However, large quantities of industrial wastes are still being discarded, thus causing environmental pollution.
As waste treatment technology suitable for environmental conservation to replace conventional incineration systems, gasification and combustion systems which combine gasification and high-temperature combustion have been developed, and some of them are about to be put to practical use.
Among the developed gasification and combustion systems, there are a system having a vertical shaft furnace as a gasification furnace (hereinafter referred to as an "S system") and a system having a rotary kiln as a gasification furnace (hereinafter referred to as an "R system").
According to the S system, a drying/preheating zone at a temperature ranging from 200 to 300.degree. C., a thermal decomposing zone at a temperature ranging from 300 to 1000.degree. C., and a combusting/melting zone at a temperature of 1500.degree. C. or higher are formed as accumulated layers in the gasification furnace. Wastes and coke charged into the furnace from an upper portion thereof descend in the furnace while exchanging heat with gases which have been generated in the lower zones. The generated gases that flow upward in the furnace are discharged from the furnace, and then combusted in a subsequent combustion furnace at a temperature of about 900.degree. C. Carbonous materials generated in the thermal decomposing zone and the charged coke descend and enter the combusting/melting zone, and are combusted at a high temperature by oxygen-enriched air supplied from a tuyere to melt ash content and inorganic materials in their entirety.
According to the R system, wastes are crushed and supplied into a drum-type rotary furnace which is externally heated by high-temperature air. In the drum-type rotary furnace, the wastes are slowly pyrolized at a temperature of about 450.degree. C. Carbonous materials generated at this time are discharged from the drum-type rotary furnace, and cooled down to a temperature at which they will not be ignited. Then, the carbonous materials are pulverized and supplied to a subsequent swirling type high-temperature combustor, and in the swirling-type high temperature combustor the pulverized carbonous materials and the gases supplied from the rotary furnace are combusted at a high temperature of about 1300.degree. C. to melt ash content into molten slag.
The S and R systems suffer various disadvantages of their own as described below. In the S system, the operating cost of the shaft furnace is high because supplementary materials such as coke and oxygen-enriched air are required for maintaining the melting zone at the bottom of the furnace at a temperature ranging from 1700 to 1800.degree. C. The use of the coke poses a problem that an increased amount of carbon dioxide is discharged from the furnace. Since almost all metals contained in the wastes are melted, they cannot be recycled as ingot metal in accordance with the type of metal. It is difficult for the furnace which belongs to a type of fixed-bed furnaces to operate stably because the wastes, in various different shapes, are stacked in layers in the furnace and the combusting/melting zone is present at a lowest region of the furnace. It is very important for a fixed-bed furnace to allow gases to flow uniformly in the layers, i.e., to maintain gas permeability. But the various different shapes of the wastes prevent the gases from flowing uniformly in the layers, and this tends to cause the gases to blow through the layers or to drift. The addition of the coke as supplemental fuel serves to keep gas permeability, but this role is not enough, and hence the flow rate of the gases and the internal pressure of the furnace cannot be kept constant. Since not all the generated gases pass through a high-temperature region in excess of 1000.degree. C., it is impossible to completely prevent the generation of dioxins and furans.
In the R system, since the gasification furnace comprises a rotary furnace which is externally heated by high-temperature air, it has poor thermal conductivity and is unavoidably large in size. Further, tar produced by the thermal decomposition and undecomposed substances cover the heat transfer surface of the furnace, making its thermal conductivity poorer. It is difficult to obtain the high-temperature air which is heated up to 600.degree. C. through a heat exchange with the exhaust gases in terms of the material of the heat exchanger. The generated carbonous materials are discharged from the rotary furnace, then pulverized, and supplied to a combustion furnace in which they are mixed with gases supplied directly from the rotary furnace and combusted at a high temperature. Therefore, the R system needs handling facilities for discharging, cooling, pulverizing, storing, and supplying the carbonous materials. Heat loss caused by cooling of the carbonous materials or heat radiation from the carbonous materials during handling thereof is not desirable from the standpoint of effective energy utilization. If the carbonous materials are discharged without being cooled, then they will be ignited by contact with air.
As described above, there have been proposed various types of systems for gasifying wastes and thereafter combusting generated substances at a high temperature to decompose dioxins and melt ash content into molten slag. However, no practically feasible technology yet has been available for recovering combustible gases by gasification from the chemical recycling viewpoint.
On the other hand, CO (carbon monoxide) and H.sub.2 (hydrogen) are widely used as gases for materials of chemical synthesis. Carbon monoxide is used for chemical synthesis of gasoline, alcohol, organic acid, and ester. Hydrogen is used for chemical synthesis of ammonia (NH.sub.3) or methanol, hydrogenation desulfurization, hydrogenolysis, fat hydrogenation, and welding. Carbon monoxide has been produced by gasification of coal or coke, and hydrogen has been produced by either steam reforming of natural gas or naphtha, or gasification of petroleum, coal or petroleum coke. Since most of those materials for producing CO or H.sub.2 are dependent on importation from abroad, there has long been a need for materials for procuring CO or H.sub.2 which are inexpensive and available without importation.